Sympathy for Cold Molecules
Physicists have cooled single atoms and molecules with two or three atoms to just a few thousandths of a degree above absolute zero, but it has proved hard to push larger molecules below about 10 degrees Kelvin. In the 15 December Physical Review Letters, researchers report cooling large organic molecules to less than 0.1 K using a technique that should also work with proteins and other biomolecules. Temperatures this low will allow scientists to study molecular structures and chemical processes with unprecedented precision and perhaps explore novel issues of fundamental physics.
In spectroscopy, researchers measure the characteristic frequencies at which molecules emit or absorb light in order to investigate their structure. Low temperatures improve the precision of spectroscopy by reducing the motion and vibration of the molecules, thereby reducing the extent to which the Doppler effect changes light frequencies.
Precise spectroscopic measurements might also allow physicists to test the limits of current theories, says Bernhard Roth of the Heinrich-Heine University in Düsseldorf, Germany. For example, he suggests comparing the energies of spectral features in molecules that are identical except for being mirror images of each other. No one has ever found such an energy difference between left-handed and right-handed molecules–a so-called parity violation. But this indirect effect of the weak nuclear force might show up if the experiment were sensitive enough.
Cooling large molecules is difficult because they can move and wriggle in many different ways, each of which must be brought under control. Roth and his colleagues chilled a dye called Alexa Fluor 350 (AF), a molecule with 42 atoms, using a technique called sympathetic cooling. They trapped positively charged AF ions in a geometrical arrangement of electromagnetic fields, then added barium ions to the trap. To cool the barium ions, they used a standard technique in which laser illumination of a chosen frequency forces the ions to repeatedly absorb and emit photons, losing a little energy on each exchange.
Because both the barium and AF ions are positively charged, they repel each other. The lighter barium ions accumulated at the core of the trap, while the AF ions formed a shell around them. As the warmer AF ions moved about, they jostled the cooler barium ions via the repulsive force between the two types, transferring energy to them. “The heat taken out of the molecular motion is permanently taken out by the laser,” Roth explains. In a matter of minutes, the barium ions reached 25 millikelvin (mK), while the AF ions got to 95 mK.
The technique can be applied to any molecule that can be charged, Roth says. “In the future, we’d like to cool even heavier molecules, such as proteins or polymers.” The cold molecules can stay in the trap for hours or even days at a time, allowing researchers to study the progress of certain internal processes, such as some very slow electronic transitions. “This is a very nice paper,” says Hans Schuessler of Texas A&M University in College Station, whose group has cooled fullerenes to about 14 K with a similar technique. “It has been an interesting subject for a long time, but now it comes slowly to fruition.”
Corinna Wu is a freelance science writer in Alexandria, Virginia.